This application includes material disclosed in the following applications assigned to the assignee of this invention. The entire contents of each of these co-pending applications are incorporated herein by reference.
(1) Application No. 09/466,313 filed Dec. 17, 1999, entitled xe2x80x9cBand Pass Filter from Two Notch Filtersxe2x80x9d.
(2) Application No. 09/491,998, filed concurrently herewith, and entitled xe2x80x9cBand Pass Filter from Two Filtersxe2x80x9d.
This invention relates to a filter circuit and, in particular, to an analog band pass filter having a more uniform group delay than such filters in the prior art.
Band pass filters have been used, alone or combined, in a host of applications virtually since the beginning of the electronic industry. The continuing problem in any application is providing a band pass filter having the desired frequency response. It is known in the art that a band pass filter can include a pair of series coupled resonant circuits that are xe2x80x9cde-tunedxe2x80x9d, i.e. have slightly different resonant frequencies. See for example, xe2x80x9cRadio Engineeringxe2x80x9d by Terman, McGraw-Hill Book Company, New York, 1937, pages 76-85.
Today, a band pass filter can be implemented in any one of several technologies. For example, passive analog filters utilize resistors, capacitors, and inductors to achieve the desired frequency response. Active filters add one or more operational amplifiers to prevent a signal from becoming too attenuated by the passive components and to exaggerate or to minimize a particular response by controlled feedback. Switched capacitor circuits are basically analog circuits but divide a signal into discrete samples and, therefore, have some attributes of digital circuits.
Finite Impulse Response (FIR) filters are completely digital, using a shift register with a plurality of taps. An FIR filter generally has a linear phase versus frequency response and a constant group delay. As such, FIR filters find widespread use in digital communication systems, speech processing, image processing, spectral analysis, and other areas where non-linear phase response is unacceptable.
A problem using FIR filters is the number of samples versus the delay in processing a signal. In order to obtain a high roll-off, i.e. a nearly vertical skirt on the response curve, a very large number of taps is necessary. Although the group delay is constant, it is relatively large, ten to fifteen times that of an analog filter, because of the large number of taps. Another problem with FIR filters is ripple, which typically exceeds 3 decibels (dB). There are other digital circuits that could be considered filters but these circuits either do not operate in xe2x80x9creal timexe2x80x9d or have such long processing times that the delays limit the utility of the techniques.
It is known in the art to use delay equalizers to improve the uniformity of the delay of a band pass filter; e.g. see xe2x80x9cElectronic Filter Design Handbookxe2x80x9d by Williams and Taylor, Third Edition, McGraw-Hill, Inc., 1995, pages 7.21-7.27 and 7.30. The effect on frequency response of adding such equalizers is not described.
Obtaining a sharp roll-off from an analog filter is often difficult, particularly for narrow band filters, e.g. one third octave or less. Even with active elements, good filters tend to be complex and, therefore, expensive. As noted above, FIR filters can provide a sharp roll-off but typically suffer from long group delay, making an FIR filter unsuitable in telephone systems, for example.
Frequency response, phase shift linearity, group delay, ripple, and roll-off are characteristics of all filters, whether or not the characteristic is mentioned in a particular application. The Q, or sharpness, of a filter circuit is often specified as the ratio of the center frequency to the band width at xe2x88x923 dB. A problem with this definition is that the roll-off on each side of the center frequency is assumed to be symmetrical (when amplitude is plotted against the logarithm of frequency). Another assumption is that the skirts of the response curves of two filters are similar. If the assumption is not valid, then comparing one filter to another becomes difficult.
In view of the foregoing, it is therefore an object of the invention to provide an analog band pass filter having short, relatively constant, group delay
Another object of the invention is to provide an analog band pass filter that is relatively inexpensive despite improved performance when compared with filters of the prior art.
A further object of the invention is to provide an analog band pass filter having higher Q than such filters in the prior art.
The foregoing objects are achieved in this invention in which an electrical signal is applied to a band pass filter, a first notch filter, and a second notch filter in any order. The center frequencies of the notch filters straddle the pass band of the band pass filter. The notch filters improve group delay and steepen the skirts of the response curve of the band pass filter. The invention can be implemented with analog filters, IIR (Infinite Impulse Response) filters, bi-quad filters, or switched-C filters.